Mix capsicums, zucchini, onion, eggplants, mushrooms and 1 tablespoon oil in a large bowl. Cook vegetables on barbecue grill plate until browned and tender, turning occasionally to cook evenly.
Place warm vegetables in a serving bowl. Add treviso, olives, vinegar, oregano and remaining 2 tablespoons oil. Toss gently to combine.
Grill or barbecue an extra quantity of vegetables. Cool; place in a glass container. Pour in olive oil to cover. Seal jar and store in a cool place away from direct light. Serve as part of an antipasto plate.
Mushrooms contain very few kilojoules (calories) and are virtually fat-free. To get the maximum nutrients, store mushrooms for no more than five days in the refrigerator.